Plasma torch with extended life electrodes

ABSTRACT

A plasma torch electrode has a copper outer shell on an inner portion of which is provided a more durable composition of a silver alloy by pressing or casting to achieve longer lifetime under arcing conditions in air or oxygen while minimizing the cost of materials and fabrication.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to plasma torches such as for heating a gas andparticularly to plasma torch electrodes, their composition, and methodsof manufacture.

Plasma torches to be improved by the present invention typically containtwo tubular shaped, water cooled, electrodes colinearly arranged alongan axis. In direct current operation, one electrode is at a highpotential and the other is normally at ground potential. There is asmall gap, typically about 1 mm, between adjacent ends of the electrodeswhere an arc is initiated during startup. Gas to be heated is forcedthrough this small gap into the inside of the tubular electrodes,thereby causing the arc to be extended into their inside diameter. Fieldcoils surrounding the electrodes cause the arc to rotate within theelectrode bores at a high velocity. The cold gas, coming through thesmall gap and then through the rapidly moving arc, is thus heated by thearc.

One electrode is referred to as the upstream electrode and normally hasa closed end and is normally the electrode to which a high potential isapplied. The other electrode, at ground potential, has an open end fromwhich the heated gas passes and is referred to as the downstreamelectrode. The heated gas may be utilized for any number of heatingpurposes including chemical processes such as ore reduction.

Further background on relevant torches may be found in U.S. Pat. Nos.3,705,975 and 4,214,736, whose description as to the construction andmanner of operation of plasma torches is incorporated herein byreference.

Electrode life, particularly at the upstream, high voltage electrode, isa concern with the foregoing and similar torch designs, particularlywhen operating with an oxidizing gas such as air as the torch gas withcopper electrodes.

As a consequence, a limited life of the electrodes for a given powerlevel and torch size has limited the use of torches in commercialapplications.

Another important factor is that in most industrial torch applications,the replacement of worn electrodes results in significant lost operatingtime for the process. Hence, longer lasting electrodes are desirableeven with somewhat added cost for such electrodes.

During normal direct current operation on copper electrodes with theupstream electrode being the anode, the life of the upstream electrodemay be less than about 100 hours and the life of the downstreamelectrode may be less than 300 hours. Oxide particles coming from theupstream electrodes tend to cause unstable torch operation. Copper oxideis stable at high temperature. These small particles enter the gapbetween electrodes, causing periodic short circuits and damage to thegap area. Reversing the polarity does not avoid the problem. Torchoperation on alternating current alleviates the gap shorting problemsomewhat but the electrode life of the two electrodes is merely madesubstantially equal at about 200 hours or less.

While copper has been the commonly used electrode material (typicallyOFHC copper with purity greater than 99%), exhibiting theabove-mentioned wear problems, some longer life torch electrodes havebeen made of silver and copper alloys in the range of 72% to 90% silver.While the use of electrodes of such a composition has been foundfavorable in terms of lifetime when operating on air or oxygen, theexpense of the electrodes has prohibited very widespread use. Therelatively high cost results both from the cost of the silver electrodematerial itself as well as from the required fabrication operations.

Some electrodes in small torches made by Westinghouse have consistedentirely of a silver-copper alloy of the eutectic composition of 72%silver-28% copper. The electrodes were made by extruding the materialfrom a rod. In other work reported by C. B. Holden of PPG Industries,Inc. in a paper "Electrode Life in An Arc Heater" (publication citationnot known), life problems of electrodes are reported and discussion ofthe characteristics of silver alloy electrodes is given. The 72%-28%silver-copper alloy was recommended; certain commercial arc heaterelectrodes were made of the 80%-20% silver-copper alloy. Both the anodeand cathode had a copper ring brazed onto one end to permit a threadedconnection. Also, it is reported that a step joint and silver solderwere used to fit deteriorated electrodes with new noses to replace thedamaged area of the same 80%-20% alloy. In the case of some rearelectrodes, this joint technique is also reported to have been usedusing a length of silver alloy tubing where the arc attachment wasexpected and copper tubing at both ends. The silver alloy tubing usedfor these electrodes was of cast material. It is mentioned that at theend of their lifetime of 5,000-10,000 hours (with an arc drawing about550 amperes), they could be repaired with a new section of silver alloytubing replacing the eroded part, giving even greater length of usefullife.

The foregoing results in considerable material cost and, also, concernabout the integrity of soldered joints which are required to be watertight. In some torches of particular current interest, the current drawnis in the range from about 1000-2000 amperes which aggravates theproblem of electrode life.

In general, silver electrode material is typically more expensive thancopper by a factor of about 30. Further, the fabrication of silver intothe shape required for manufacturing electrodes might double thisunfavorable ratio. Actual test data measuring wear on an anode indicateselectrode life extended by factors of about 7 to 10 times in the highwear region of the electrode surface when using silver alloy material ascompared to copper. An objective of the present invention is to providedesigns for electrodes and their fabrication that are sufficientlyeconomical so that the cost disadvantage does not greatly offset theimprovement in life time.

In accordance with the present invention, a torch electrode comprises atubular outer shell of a first material such as copper. On the innersurface of the outer shell, or preferably merely a portion of the innersurface, is directly fabricated an arcing portion of a second, moredurable, metal such as silver-copper alloy. The second metal isprovided, at least, in the region where the arc normally attaches to theelectrode surface under the operating conditions to be encountered. Inone method a silver alloy powder is compacted onto the shell by a hotisostatic pressing process. In another method, the silver alloy in theform of a powder or other form such as a wire can be placed in a cavitybetween the shell and a liner and then melted in a furnace to form acast layer of alloy in the proper location. By such techniques, theoccurrence of the silver alloy can be minimized both in axial extent aswell as in thickness. A silver alloy thickness of no greater than about6 mm, on the copper outer shell, is sufficient to provide a lifetimeextension of about 7-10 times as compared to copper with an economicalcost. The silver alloy thickness is generally no more than about half ofthe total electrode thickness. This is to extend life with lowermaterial cost. A complete electrode, or complete thickness of silveralloy provides only a marginally greater improvement in life but at aconsiderably greater cost. While significant advantage can be taken ofsuch electrodes as provided in a unitary integral structure, it is alsoa suitable design to provide the copper shell in detachable sections, asby having threaded ends, with the use of O-ring seals as desired, inorder to permit replacement of only a section of the shell when thesection having the arcing portion becomes worn.

THE DRAWINGS

FIG. 1 is a general view of a plasma torch improved in accordance withthe present invention by one embodiment;

FIG. 2 is a cross-sectional view of an embodiment of the presentinvention at a preliminary stage in its fabrication;

FIG. 3 is a cross-sectional view of the embodiment of FIG. 2 with itsfabrication completed;

FIG. 4 is a cross-sectional view of an electrode in accordance withanother embodiment of the present invention at a preliminary stage inits fabrication; and

FIG. 5 is a cross-sectional view of an electrode assembly in accordancewith another embodiment of the invention.

PREFERRED EMBODIMENT

As shown in FIG. 1, a plasma torch in accordance with the presentinvention typically contains two tubular shaped electrodes 10 and 12colinearly arranged along an axis. The electrodes are provided withwater cooling equipment 14 on their outer surface (not detailed herein).One electrode 10 has a closed end 16 and is referred to as the upstreamelectrode; it is normally operated at a high positive potential relativeto the downstream, open ended electrode 12 that is normally at groundpotential; power being supplied by a power supply means 18. There is asmall gap 20, typically about 1 mm, between adjacent ends of theelectrodes where an arc is initiated during startup when the electrodesare energized by the power supply. Gas to be heated, supplied from a gassource (not shown) is forced through this small gap into the insidediameter of the electrodes, causing the arc to be extended into theelectrode inner space. Field 22 and 24 coils surrounding the respectiveelectrodes cause the arc to rotate within the electrode bores at highvelocities. The cold gas, coming through the small gap and then throughthe rapidly moving arc, is thus heated by the arc. The gas continues outof the bore of the downstream electrode (to the right in the FIGURE)where it can be utilized for any process. Further information withrespect to the construction and operation of the basic torch is wellknown, such as in above-mentioned U.S. Pat. No. 3,705,975.

By the present invention the high voltage electrode 10 has an outershell 30 of a first conductive material, such as copper, that extendsthe axial length of the electrode and an inner arcing portion 32 of asecond conductive material such as silver or a silver copper alloy thatis more durable in the gas with which the torch is operated. The arcingportion 32 may be confined to a region of the electrode that is mostaffected by the arc under the operating conditions of the torch.Furthermore, the thickness of the second material in the arcing portionmay be limited to a thickness of no more than about half the electrodethickness, such as about 6 mm. Thus, the quantity of the second materialas compared to that of the less expensive, first material isconsiderably less.

The invention may also be practiced in torches in which both of the twoelectrodes have the construction employing the limited surface areaarcing portion 32 in accordance with this invention. This would bedesirable when operating on alternating current, for example.

The outer shell 30 is principally of copper as fabricated substantiallyin accordance with prior practice for plasma torch electrodes. Theinner, arcing portion 32 of the second, more durable, metal may be anyof a Wide range of compositions including silver and silver alloys whenoperating in air. Among the suitable compositions are silver-copperalloys ranging from the eutectic of 72% silver-28% copper, by weight, toabout 80% silver-20% copper. In part, the composition selection isdependent upon the particular method of fabrication chosen as will beexplained further hereinafter. Any such compositions may containadditional constituents, such as tungsten, to provide even longer wearin air.

FIG. 2 shows one fabrication technique for the improved electrode. Theouter shell 30 is arranged with an inner liner tube 40, which, forexample, is of copper having a thickness of only about 2 mm. The linertube 40 is joined to the outer shell by weld joints 42 at theirrespective ends. The outer shell and liner tube are configured so as toprovide an accessible volume 44 therebetween. In the example of FIG. 2,the outer shell is recessed from its maximum thickness in the area wherethe arcing portion is to be fabricated and the liner tube is of morerestricted inner diameter in that portion of the structure. Afterassembly of the liner tube, the volume 44 between the outer shell andthe liner tube is filled with alloy metal for the arcing portion. InFIG. 2, the space is filled with an alloy powder 46 of chosencomposition as aforesaid. Then the assembly is treated to form an arcingportion of the second metal of greater durability to arcing than thefirst conducting metal from which the shell is formed. In the case ofthe assembly of FIG. 2, the treating is in the form of hot pressing,such as hot isostatic pressing, in order to compact and fuse thepowdered metal into relatively dense, substantially void free, material.Before pressing, a filling and evacuating tube 48 is used to supply thepowdered material 46 to the inner volume, to remove air from that space,and to seal off the volume 44.

Subsequent to the performance of the pressing operation, the liner 40and the inner surface portion of the arcing portion is machined away toa uniform diameter of the outer shell 30 and the arcing portion 32 whichnow is dense, fused metal, as shown in FIG. 3.

In an alternative form of the invention as shown in FIG. 4, the linertube 40' is configured of a consistent inner diameter and is joined atjust one end by a weld joint 42 to the outer shell leaving an opening 50at the opposite end for access to the volume 44' between the liner tubeand shell. The second metal, such as silver-copper alloy, is supplied tothat volume 44' such as either in the form of powdered material orpieces of wire or the like and then the assembly is subjected to heatingresulting in molten alloy 52 which is then cooled to form a cast layerin the proper location on the shell. After that the liner is removed andthe surface smoothed.

The liner 40 in FIG. 2 is configured to allow for compaction, which isnot necessary for the casting operation of FIG. 4.

In forming a cast arcing portion 32 according to the method depicted byFIG. 4 various alloy compositions may be used but it is believedfavorable to use a noneutectic composition, even though the eutectic issuitable. The reason is that a non-eutectic, such as 80% Ag-20% Cu,instead of the eutectic, 72% Ag-28% Cu, is much less likely to formshrinkage voids during solidification from the molten state to the solidstate.

FIG. 5 shows an alternative design where the shell portion 30a on whichthe more durable arcing portion 32 is pressed or cast is joined to oneor more other shell pieces 30b of the first metal, copper. For thispurpose, the different shell sections 30a and 30b have interfittingthreaded elements 60 for joining them and O-ring seals 62 at theirjoints. In the embodiment shown in FIG. 5, only the central section 30aof the outer shell is provided with the improved arcing portion 32. Whenthe arcing on this portion reaches a wear limit, it alone need bereplaced rather than the whole electrode, thus realizing additionalsavings.

It is therefore seen that unique processes for manufacturing plasmatorch electrodes are provided that result in a substantial increase inoperating life compared to conventional copper electrodes whileminimizing the material cost and fabrication cost attendant to providingan arcing portion more durable than copper. From the examples given, itis believed that the inventive concepts may be practiced in still otherforms as will be apparent to those skilled in the art.

We claim:
 1. A plasma torch comprising:first and second tubularelectrodes arranged substantially colinearly along an axis with adjacentends defining a gap therebetween; means for initiating an arc acrosssaid gap; means for supplying a gas to be heated through said gap intospace within said tubular electrodes and causing the arc to extend torespective arcing portions of said electrodes' inner surfaces that areless than the entire extent of said inner surfaces; means for rotatingthe arc so it moves circumferentially about said electrodes' surfacesand heats gas therebetween; one of said electrodes being open to allowheated gas to exit the torch; at least one of said first and secondelectrodes comprising a shell consisting principally of a firstconductive metal and said arcing portion consisting principally of aconductive metal that is more durable than said first conductive metalunder the arcing conditions of the heated gas located on an innersurface of said shell.
 2. A plasma torch in accordance with claim 1wherein:said shell of said at least one electrode consists principallyof cooper and said arcing portion thereof consists essentially of asilver-copper alloy.
 3. A plasma torch in accordance with claim 1wherein:said at least one electrode is said first electrode operated ata high DC voltage relative to said second electrode.
 4. A plasma torchin accordance with claim 1 wherein:each of said first and secondelectrodes has a construction comprising a shell of a first conductivemetal and said arcing portion of a conductive metal that is more durablethan said first conductive metal.
 5. A plasma torch in accordance withclaim 2 wherein:said silver-copper alloy consists essentially of about72% to about 80%, by weight, silver.
 6. A plasma torch in accordancewith claim 5 wherein:said alloy consists essentially of a eutecticsilver-copper alloy having about 72% silver and about 28% copper.
 7. Aplasma torch in accordance with claim 5 wherein:said alloy consistsessentially of a non-eutectic silver-copper alloy having about 80%silver and about 20% copper.
 8. A plasma torch in accordance with claim1 wherein:said arcing portion of more durable metal has a thickness upto about half the total thickness of the electrode including the arcingportion and shell.
 9. A plasma torch in accordance with claim 3wherein:said first electrode is upstream relative to said secondelectrode which is open to allow heated gas to exit the torch.
 10. Aplasma torch in accordance with claim 4 wherein:said electrodes areoperated at an A.C. voltage.
 11. A method of making a plasma torchelectrode comprising:providing an outer, tubular, shell of a firstconductive metal; joining an inner liner tube to said outer shell withan accessible volume therebetween; supplying said volume with othermetal of a composition differing from said first conductive metal toform an assembly; treating said assembly to form an arcing portion of asecond conductive metal of greater durability to arcing than said firstconductive metal from the metal with which said volume is filled.
 12. Amethod in accordance with claim 11 wherein:said other metal is powderedand the treating is performed by hot pressing the assembly.
 13. A methodin accordance with claim 12 wherein:after hot pressing said liner isremoved.
 14. A method in accordance with claim 13 wherein:said arcingportion is machined to the same diameter as an exposed inner surface ofsaid outer shell.
 15. A method in accordance with claim 11 wherein:thetreating is performed by heating to melt the metal within the volume andthen cooling the molten metal to form a cast layer.
 16. A method inaccordance with claim 11 wherein:said other metal consists principallyof silver or a silver alloy.
 17. A method in accordance with claim 11wherein:said other metal consists essentially of a silver-copper alloy.18. A method in accordance with claim 11 wherein:said other metalconsists essentially of a silver-copper alloy having about 72% to about80% by weight, silver.
 19. A method in accordance with claim 11wherein:said other metal consists essentially of a eutecticsilver-copper alloy having about 72% silver and about 28% copper.
 20. Amethod in accordance with claim 11 wherein:said other metal consistsessentially of a noneutectic silver-copper alloy having about 80% silverand about 20% copper.